1. Field of the Invention
The invention relates to organosiliconate powders, to a process for producing them, and to their use, especially for hydrophobizing mineral building materials.
2. Description of the Related Art
Alkali metal organosiliconates have already been used for decades for hydrophobizing—that is, for imparting water repellency to building materials. Generally speaking, these are inorganic building materials which may be silaceous or nonsiliaceous in nature. Aqueous solutions of methylsiliconate, in particular, have a great significance here, particularly the potassium derivative (potassium methylsiliconate) or the sodium derivative (sodium methylsiliconate). On account of their high solubility in water, they can be applied in the form of an aqueous solution to solids, where evaporation of the water is accompanied by the formation of durably water-repellent surfaces which adhere firmly under the influence of carbon dioxide. Since they contain virtually no hydrolytically eliminable organic radicals, curing takes place, advantageously, without release of unwanted volatile organic by-products.
Aqueous solutions of organosiliconates are especially suitable for hydrophobizing weakly acidic to weakly alkaline building materials, more particularly products comprising fired clay, natural stone, cement or gypsum. The hydrophobizing agent may be applied here either by impregnation or mass hydrophobizing. In the case of impregnation, for example, products of fired clay or natural stone are immersed for a certain time into an aqueous dilution of the organosiliconate or are sprayed with a dilution of this kind, the active substance in solution in water penetrating the porous microstructure of the building material by capillary action. After a time of a few minutes or several hours or even a number of days after drying of the building material, depending on the prevailing conditions, a hydrophobic zone is developed which surrounds the building material and drastically reduces its capillary water uptake. In the case of mass hydrophobizing, the aqueous solution of the organosiliconate is mixed, after further dilution where appropriate, with the aqueous slurry of a building material. Measurements of the water uptake of the building material after it has set and dried show a greatly reduced water uptake as compared with the unhydrophobized building material.
For example, U.S. Pat. No. 2,803,561 describes the use of aqueous solutions of organosiliconates and of methyl siliconate powder for hydrophobizing calcium-containing masonry (gypsum, limestone).
The preparation of alkali metal organosiliconates, especially potassium or sodium methylsiliconates, has been often described. In the majority of cases, the focus is on producing aqueous solutions which are ready for application and are stable in storage.
For example, EP 650968 describes a continuous process starting from organotrichlorosilanes and proceeding via the organotrialkoxysilane as an intermediate. Advantageous features of that process are that the alcohol and hydrogen chloride by-products formed are recovered and that the siliconate solution formed is virtually chloride-free.
The advantage of the mass hydrophobizing of gypsum or cement, for example, is that the building material not only is surrounded by a hydrophobic zone but is water-repellent through and through. This is especially important with building materials which have a tendency to be water-soluble, such as gypsum, or if the building material is cut into pieces after the water repellency treatment. This technique is employed, for example, in the production of gypsum plasterboard panels, gypsum wallboarding panels or gypsum fiberboard panels. Plasters and filling compounds or tile adhesives, however, are supplied to the building site as powders, in bags or silos, and are made up with water by stirring on site. For application in gypsum- or cement based plasters, filling compounds, repair filler powders, tile adhesives and similar mineral building materials, therefore, a solid hydrophobizing agent is required that can be added to the ready-to-use dry mixture and which develops its hydrophobizing effect in a short time only on addition of water during application on site, such as on the building site, for example. This is called dry-mix application.
The majority of conventional dry-mix hydrophobizing agents in accordance with the current state of the art are supported systems, which means that a hydrophobizing agent which is in fact in liquid form, such as an active silane and/or siloxane ingredient, for example, is applied to a support material which is more or less chemically inert. The amount of hydrophobizing agent applied in this case is only such as to produce a dry and free-flowable powder. This produces active contents of only 30-50%—it follows from this that the mass of the inactive support material accounts for 50-70% of the total mass. The support material may be inorganic—examples are silicas and silicates—or organic—examples are polyvinyl alcohols, as described in WO 2010052201. By combination with the water used for making up the mix, and by intensive mixing, the liquid hydrophobizing agent develops its effect, while the support material remains in the cured building material as a functionless filling material. The support material may even have adverse effects on the fully cured building material; it is known, for instance, that polyvinyl alcohols tend to increase the hydrophilicity of gypsum building materials, which is counterproductive.
Conventional dry-mix hydrophobizing agents such as salts of fatty acids have a series of disadvantages. With these known products, a problem which occurs is that the high hydrophobicity of the powders and premature migration of the hydrophobizing agent onto the building material which is still to be mixed with water results in a delayed initial miscibility. As a result, in addition to the loss of time, unwanted dust is formed from the building material as a result of the delayed wetting with water. Likewise, conventional dry-mix hydrophobizing agents based on siloxane have a comparatively low active content, because they usually consist of a liquid active siloxane ingredient on a solid support, as described in WO 2010052201, example 1. Apart from its support activity, the support has no importance, and an increase in the active content would lead to sticky dry-mix hydrophobizing agents which would no longer be free-flowable. As a consequence, these hydrophobizing agents are not efficient enough.
The highest active ingredient contents are obtained with unsupported systems, such as the pure siliconate powders described in U.S. Pat. No. 2,803,561, for example. They are suitable in principle as dry-mix additives. For example, DE A 10107614 describes cement-based tile adhesives made water-repellent through addition of 0.1% to 20% of alkali metal siliconate as dry powder. In contrast to the oleates or stearates commonly used, the alkylsilicic acids which form in the building material are said to ensure water vapor permeability and so not to hinder the drying of masonry or screed. Mention is made of alkali metal alkylsiliconates with methyl radicals, ethyl radicals, and also with the various isomeric propyl radicals and butyl radicals. A disadvantage of these additives is that the most effective representatives, with more than two C atoms in the alkyl radical, are obtainable only by way of the expensive intermediate step of a hydrosilylation. In contrast, methylsiliconates, which are available much more cost-effectively from methyltrichlorosilane, a byproduct of the Müller-Rochow process, are not durably water-repellent, especially in strongly basic, cement-based building materials, on account of their solubility in water.
U.S. Pat. No. 2,898,221 as well describes alkali metal alkylsiliconates with methyl, ethyl, propyl, vinyl, or allyl radical as additives for concrete. The siliconates, which can also be used in solid form (column 1/line 43), not only raise the hydrophobicity, but also, in fractions of 0.05%-0.7%, based on the Portland cement employed, give the building material a higher compressive strength, although a retarded setting behavior is likely (column 2/line 2). Mixtures of different siliconates may also be employed (column 1/line 56).
According to DE 2245927, aqueous solutions of alkali metal propylsiliconates, both alone and in blends with other alkyl siliconates, more particularly methyl siliconates, are particularly effective surface water repellency agents for strongly basic building materials which have not yet fully set. They are prepared in two stages by cohydrolysis of propyltrichlorosilane with other alkyltrichlorosilanes, followed by dissolution in aqueous alkali. Comparative examples with methylsiliconate and ethylsiliconate show their low permanence in the water repellency effect on concrete. From the prior art described it is evident that cost-effective and efficient dry mix additives on a siliconate basis, for universal use both in neutral and in basic building materials, have not so far existed. The methyl siliconates obtainable from the inexpensive methyltrichlorosilane are less well suited to applications in strongly basic environments, such as cement, on account of their ready solubility in water, this poor suitability being evident from a low permanence of the water repellency effect. Conversely, the siliconates that are more effective in cementitious systems, with radicals larger than methyl, have to be prepared via an additional operating step, an expensive hydrosilylation reaction with an olefin as further raw material and, preferably, with the use of expensive platinum catalysts. Mixtures of methyl siliconates and siliconates with larger radicals, besides the extra expense of a dual operating chain from production to warehousing, possess a further great disadvantage: as the size of the organyl radical goes up, there is a decrease in the wettability of the siliconate powder by water, and so even small fractions in the ready-to-apply building material can lead to the abovementioned problems such as retarded mixability and formation of dust on mixing with water.